New agents that reduce normal tissue toxicities or enhance the efficacy of radiation or chemotherapy are urgently needed, to reduce complications as well as to allow full treatment and a higher probability of cure. The adverse effects of anticancer treatment on development in the very young are especially feared. We have shown that radiation has effects in zebrafish similar to that seen in humans. The zebrafish has additional unique advantages for [unreadable] investigating the effects of "radiomodifiers" on viability and development and may be especially suited for high throughput screening (HTS). The high genetic and physiological homology of zebrafish to higher vertebrates such as mammals increases the odds that molecules bioactive in the zebrafish would be clinically relevant, while its small dimensions and ease of breeding and care renders the [unreadable] zebrafish practical for assays incorporating microplates of 96 or more wells. Because the staging of zebrafish embryos is easy and accurate, effects of radiation and radiomodifiers on specific tissues and developmental milestones can be effectively investigated. We have found that radiation results in caspase activation in embryos that is detectable via proluminescent probes and which is proportional to the degree of end-organ damage. This "mix-and-measure" assay can be performed in 96-well microplates, requires only the addition of reagents followed by minimal subsequent manipulation, and both positive and negative controls are available. The assay results in signal proportional to the radiation dose, with a high signal-to-background ratio, and associated with Z-factors > 0.5 -all properties useful for HTS. This application therefore proposes to capitalize on our preliminary and published findings to refine and develop a zebrafish based assay for HTS. We propose to: (1) develop this assay for HTS, (2) configure this assay, including determining the suitability of a separate luminescent assay that measures viability as a secondary screen for positives detected on our primary assay. These efforts together have the potential of developing an effective zebrafish-based assay of high quality and suitable for HTS for identifying novel agents for protecting the young during radiation and other anticancer treatment. [unreadable] [unreadable] [unreadable]